1. Field of the Invention
The invention relates generally to soldering equipment and more specifically to high volume production equipment capable of reflow soldering of printed circuit boards incorporating surface mount technology.
2. Description of the Prior Art
Temperature stability, repeatability, and uniformity in results for reflow soldering of both lightly and heavily loaded surface mount device (SMD) printed circuit boards are becoming increasingly more important. (Roffey, et al., "Atmosphere Management in a Surface Mount Reflow Furnace," Proceedings of SMTCON West. Sept. 1990.) A dual search for better quality solder joints and environmentally safe use of gases and chemicals has lead to the development of special atmospheres for use in reflow furnaces. (See, Hwang, J., "Soldering With Controlled Atmospheres," Circuits Manufacturing, May 1990, pp.64-65 [discussion of dry air, nitrogen, hydrogen, nitrogen/hydrogen blends, dissociated methanol/nitrogen blends, dissociated ammonia, exothermic gas, and nitrogen/dopants reflow atmospheres].) Post-solder CFC cleaning can be eliminated or reduced by the use of no-clean reflow processes that combine special solder creams, reflow ovens, and atmospheres. (See, Murray, J., "No-Clean Reflow," Circuits Manufacturing, April 1990, pp.68-69; and see, Trovato, R., "Soldering Without Cleaning," Circuits Manufacturing. April 1990, pp.66-67.)
The successful implementation of SMD technology depends on first-time product yields that are as good or better than conventional through-hole techniques. (Flattery, D. K., "Selecting and Controlling Process Parameters for Successful Infrared SMT Reflow," PCNetwork, September 1988, pp.13-19.) An increased joint count from increased component densities makes first-time yields highly dependent on improvements in solder defect rates. To obtain low solder defect rates requires the selection and control of reflow parameters. The reflow process is basically a function of the time an assembly is in a reflow atmosphere and the temperature of the atmosphere. Differences in time and temperature exist for various atmosphere gases that are present. For example, Flattery asserts that to eliminate board and flux damage the temperature limit for air is 230.degree. C., and for nitrogen it is 260.degree. C. These temperature limits can be difficult to maintain at the corners of a board while attempting to raise the center of the board to minimum temperatures needed to reflow solder (205.degree. to 210.degree. C). Early infrared (IR) reflow furnaces had been converted from IR fusing machines with little success. SMT assemblies have characteristics that resist quick and uniform heating, in contrast to bare circuit boards. Heat transfer by convection, conduction, and radiation is a function of the surface area of an object. And although radiation can deliver energy directly to the interior of an object, the surface always absorbs more than anywhere else. It is inevitable that some temperature difference will exist during reflow between the various parts of an assembly, based on their mass to surface area, according to Flattery.
As mentioned above, the environment in which solder reflow takes place is an important parameter of the whole process. The atmosphere must be controlled to achieve complete process control. (Arslancan, A. N., IR Solder Reflow in Controlled Atmosphere of Air and Nitrogen," SMTCON Technical Proceedings, April 1990, pp.301-308.) The benefits of controlled atmosphere soldering comprise positive exhaust of process effluents, isolation of the process atmosphere from the variability of the room environment, and process reliability and repeatability. Additional benefits are realized by using an inert atmosphere of nitrogen. Oxidation of the board material, flux, solder paste, and metal surfaces is eliminated; meaning improved solder wetting and bonding, solder joint appearance, widened process window, and ease in residue cleaning. In the SMD technology (SMT) industry, air has been used in passive (natural convection) form and in compressed clean/dry (controlled convection) form. Air is readily and freely available, and therefore the least expensive gas to use. But air contains oxygen and other oxidizing agents, and the use of air requires fairly active fluxes to achieve good quality solder joints. Using nitrogen (N.sub.2), instead of air, offers many benefits that culminate in increased overall yields. Regardless of the kind of reflow system used, N.sub.2 atmospheres result in reductions of the defect rates, according to Arslancan.
IR reflow machines have been used in the past to solder molded circuit boards. (Chan, B. "The Evaluation of Various Infra-red Reflow Machines for use with Molded Circuit Boards," Proceedings, IEPS, September 1989, pp.188-198.) Molded circuit boards require precise temperature control because they are made from engineering thermoplastics instead of thermoset plastics. For successful soldering, it is necessary to have even temperatures across the assembly undergoing reflow. Otherwise, burning and warping of the substrate and/or components can occur.
Digital Equipment Corporation (DEC) used vapor-phase processing for about six years for mass reflow of SMT assemblies. (Kasturi, S., "Forced Convection: The Key to the Versatile Reflow Process," Proceedings, NEPCON East, June 1990, pp. 1015-1024.) DEC then transitioned to the IR process, which eliminated problems in tombstoning and thermal shock that are associated with the vapor-phase process. But then problems with the IR process, e.g., uneven and inefficient heat transfer, edge burning and long process times, led DEC to the convection reflow process. Efforts have been made by IR/convection manufacturers of reflow equipment to mix radiation and convection in ratios ranging from 30:70 to 10:90. In reflow applications, forced convection is favored because convective forces can be controlled to become the primary heat transfer mechanism, according to Kasturi. Such control can be achieved by regulating the volume of gas present in each of several zones, the temperature and velocity profiles of the gas, and its flow pattern. Kasturi uses heating elements not to heat a board directly, but to heat the gas that will subsequently carry heat to the board. As reported by Kasturi, DEC has demonstrated that a furnace using forced convection with proper control of the fluid flow has several advantages over an IR system. All of which translate to reduced furnace set-up time, increased process flexibility, and improved product quality.
Zone controlled, forced convection reflow soldering systems are regarded as embracing a viable technology for reflow soldering of all types of SMD product, especially large, dense multilayer surface mount assemblies. (Brammer, D., "Fundamentals and Process Applications of Zoned Convection Reflow," EXPO SMT International '89 Technical Proceedings, pp.169-171.) Recirculated, heated air, recirculated in sufficient volume but at low velocities, is a very efficient heat transfer method, as is reported by Brammer. A zoned convection reflow (ZCR) process uses recirculated heated air that is divided into individually-controlled zones. A ZCR furnace is similar to a zoned IR reflow furnace in many ways, except that individual zones do not use IR emitter panels or lamps. Instead, air orifices and air heater elements, panels, and baffles are used. Such a system heats about 95% by forced convection and 5% by IR. Brammer diagrams in FIG. 1 (Brammer, supra) a multiple-zone system consisting of eight or ten individual, independently-controlled zones of top and bottom heat (four or five above the conveyor belt and four or five below). Assemblies are carried by the conveyor belt into a first heating zone called the pre-heat zone. The pre-heat zone ramps the temperature up from room temperature to 100.degree. to 125.degree. C., to minimize thermal shock. Preheating drives off flux solvents and moisture, and it activates the flux. Following passage through a transition zone, the assembly travels through three or four zones of top and bottom heat, each at different temperatures depending on the required furnace profile. A slight vacuum of the exhaust in the transition zone draws byproducts away from the assembly, thus the assembly constantly encounters fresh, heated air. Reflow of the solder occurs in the last zone, therefore the solder is liquid for only a few seconds. Cooling of the assembly is accomplished in a last, forced cooling zone that has been incorporated into an off-load area. (See also, Martel, M. L., "Forced Convection: The Dark Horse," Circuits Manufacturing, February 1989, pp.27-40 [describes Heller 1148R and SPT 770 convection reflow soldering systems and surveys the then-current state-of-the-art]; and see, Martel, M. L., "New Wrinkles in Reflow," Circuits Manufacturing, June 1989, pp.33-41 [survey of various new developments in convection reflow techniques].)
A suggested optimal temperature profile for preheat, dry, reflow, and cooling phases is diagrammed by Cox. (Cox, N. R., "Convection in the IR Furnace: It's Not Just for Panels," Circuits Manufacturing, September 1989, pp.24-27 [FIG. 2].) Such control in a ZCR system is implemented by using a rapid-response thermocouple, e.g., as in FIG. 1, supra. The key objective of a reflow system is to control the time/temperature profile accurately and consistently, and to accomplish reflow soldering in as brief a period as is possible.
The addition of forced convection to reflow techniques has significantly reduced temperature distribution effects associated with both lamp and panel IR systems. The work of Brammer and Kasturi demonstrate that using convection as the only method of heating results in even better temperature distributions, and low component temperatures. An "atmosphere tight" system should be capable of operating with a maximum oxygen content of less than five parts per million (ppm) above that of the source gas. Nitrogen has many benefits as a cover gas and the use of toxic gases in some prior art systems may prove to be flammable and/or harmful to the environment.